1. Field of the Invention
This invention relates to apparatus used in an encoding system of the type that utilizes acoustic energy to produce a code representative of a selected keyboard position, for example, in business equipment such as typewriters, teletypewriters, calculators, adding machines, cash registers and the like. More particularly, the present invention relates to acoustic transmission members used to transmit sound waves induced by the key actuated strikers of a keyboard in acoustic encoding systems. The present acoustic transmission member is an improvement over previously known straight acoustic rods disclosed incopending application identified by U.S. Ser. No. 853,778 filed Nov. 21, 1977, now abandoned, entitled "Acoustic Encoding Apparatus" and having the same assignee. To the extent appropriate to the present invention, the disclosure of the above-identified copending application is incorporated herein by reference.
2. Description of the Prior Art
Encoding apparatus for use with many and varied types of equipment have long been known. Most recently, encoding apparatus employ non-mechanical techniques such as electronic components to measure i.e., sound, light or heat and to provide systems capable of meeting modern needs. These encoders successfully enable production of machines that are less bulky, lighter in weight, more economical and reliable. In addition they provide greater operating capabilities in terms of providing more available functions, than machines having primarily mechanical arrangements. Still, there is a continued search for improvements in encoding systems to enhance their economic value and to increase their reliability, especially on a mass production basis.
Encoding apparatus based on acoustic energy and specifically those utilizing sound waves generated from a keyboard input have been briefly disclosed in IBM Technical Disclosure Bulletins including, Arosenius, Vol. 14, No. 10, March 1972 and Lisk, Vol. 20, No. 1, June 1977. More specific approaches are fully disclosed in the above-mentioned copending application U.S. Ser. No. 853,778 (referred to hereinafter as prior application '778, for brevity) and in United Kingdom Pat. No. 1,386,070.
The basic principle employed in acoustic encoding apparatus of the type mentioned is to provide an output, usually in the form of a code, established by measuring the elapsed time between diverging sound waves induced at a point of origin within an acoustic member in response to actuation of a selected key. Devices to measure the elapsed times comprise a counting device to define the time interval between arrival of the diverging sound waves at spaced sensing elements connected to each end of the acoustic member. The elapsed time for each striker position along the acoustic member is unique and the code produced is adaptable to control operation of the machine according to the key selected.
While a considerable improvement in the art of encoders has taken place, prior art acoustic keyboards have encountered significant limitations. Though in theory a unique elapsed time is presented for each striker positioned along the acoustic member, these times are subjected to tolerance factors other than those in devices used to measure the time interval that may affect accurate code response. For example, it has been found that acoustic encoders are sensitive to variations in the acoustic properties of the material selected for construction of the acoustic transmission member. These variations undesirably affect the velocity at which sound waves are transmitted through the member. The acoustic encoding process is fixed or pre set with respect to the code generated in response to the actual measured elapsed time. A significant accumulation of tolerances in the system causes an excessive interval beyond the established time between signals such that a false code is generated. A false code possibility is realized moreover upon a study of a keyboard layout which typically includes at least one straight row of keys in proximal relationship. Each actuator operates a striker to impact the acoustic transmission member and thereby induce within the member diverging sound waves. The strikers are arranged in a side-by-side relationship according to the spacing among keys of the particular keyboard. The acoustic transmission member disclosed in the prior art is straight and traverses the row of strikers. Consequently, prior art acoustic encoding systems must accurately distinguish elapsed times within a range fixed by the spacing between proximal strikers in a straight row.